1. Field of the Invention
The present invention relates to a method of notifying a USB host controller when a power source device supplying power to a USB device is abnormally stopped in a system using a hub device for communicating through USB.
2. Description of the Related Art
(1) Overview on USB
Universal serial bus (USB) is an interface standard that connects a personal computer (hereinafter abbreviated as PC) to various peripheral devices (hereinafter called USB devices) according to a common specification standard to realize high speed communication. The USB has a good usability; furthermore, a host controller (hereinafter called USB host) is arranged on a PC side, and communication with various USB devices is controlled by a host controller in a centralized manner, thereby allowing an interface configuration on a USB device side to be simplified and inexpensive.
The number of USB ports that a USB host can directly control is limited. Accordingly, a method is adopted that causes a USB hub device to intervene between a USB host and USB devices. In a USB system configuration, sides of a USB host and a USB device are generally called upstream and downstream, respectively. The description of the present invention also adopts these designations.
(2) Power Source Configuration of USB
Methods of supplying a USB device with power include two methods, that is, a bus power that directly supplies power via a power source line (hereinafter called VBUS line) for supplying power to a USB device of a USB host, and a self power that supplies power from an external power source other than the VBUS line. In the case of supplying a USB device with power according to the bus power, it is specified that the maximum consumption current that can be supplied to the downstream is up to 100 mA. In the case of the self power, it is specified that this current is up to 500 mA.
As to the self power, for instance, Japanese Patent Application Laid-Open No. 2002-94539 discloses a proposal of a configuration including a power source circuit having a function equivalent to that of the self power in a product incorporating a USB host without using an external power source. The USB standard requires that the source voltage range at the input end of a USB device should be from +4.75 to +5.25 V. In the case of supplying power from a USB host, a long wiring path to one of a USB hub device and a USB device increases a voltage drop due to wiring impedance. Accordingly, there is a possibility that a limitation may necessarily be imposed on the configuration of a product to meet the USB standard. Supplying power to a USB device by a power source circuit provided in a product as with Japanese Patent Application Laid-Open No. 2002-94539 has a following advantage. That is, a required consumption current that can be preliminarily expected can be supplied; a voltage conversion section that converts a source voltage into +4.75 to +5.25 V and outputs the converted voltage is installed, thereby allowing a configuration of a product meeting the USB standard to be realized.
Japanese Patent Application Laid-Open No. 2004-29893 discloses a proposal of a configuration that turns off a switch to protect a VBUS line and notify a controller when a USB device consuming current beyond expectation is connected in case of a failure. Furthermore, a switch IC that interrupts current in case where overcurrent flows into a VBUS line, and a USB hub IC that includes an enable terminal for on/off operation on the switch IC, and an overcurrent detection terminal for allowing a detection signal to be input when overcurrent in the switch IC is detected have been on the market. Combination thereof realizes a configuration that causes the switch IC to interrupt the VBUS line, activates the overcurrent detection terminal of the USB hub IC and causes the USB hub device to notify the USB host of the overcurrent detection condition, where the downstream is in an overcurrent condition.
Unfortunately, when activation of the protection function is stopped owing to a failure in a voltage conversion section itself that is provided as the self power or when a USB device in an overload condition is connected to a downstream port, a following phenomenon may occur. That is, the protection operation against overcurrent by the switch IC does not necessarily function first, and stopping may be performed by a protection function of the voltage conversion section itself. In such a case, although the USB host can recognize the protection function operation of the switch IC connected to the downstream port, this host cannot recognize a stoppage of the self power voltage conversion section. In this condition, the self power voltage conversion section is stopped. Accordingly, the VBUS power source of the USB device is interrupted. In this case, the original cause is one of a failure of the voltage conversion section and the overload condition of the USB device. However, there is a possibility that the USB host erroneously recognizes that the downstream USB device has forcedly been pulled out by a user.
In the case where the voltage conversion section is stopped owing to a protection function operation, off and on restart of an input power source is typically required for recovery. However, the USB host erroneously recognizes that the voltage conversion section has not been stopped but the USB device has been forcedly pulled out instead. Accordingly, this host does not start an automatic recovery operation. As a result, from the standpoint of a user, the USB device is kept stopped in a hung-up condition, and the USB host cannot perform appropriate processes, thereby degrading usability.
As with the proposal of Japanese Patent Application Laid-Open No. 2004-29893, it can be considered that, in the case where the self power voltage conversion section is stopped, a detection signal is transmitted to the USB host. This case is advantageous if the USB host, the USB hub device and the USB device are on the same substrate. However, in one of the case where the USB host and the USB hub device are connected by a long wiring path and the case of plural steps of USB hub devices, a signal line for transmitting a detection signal to the USB host is required to be provided in addition to a typical USB communication line. As a result, the interface configuration becomes complex, thereby increasing the cost.